In special lighting applications, in particular in traffic signalling, the light is to be emitted according to a pattern that corresponds to positions where the user needs to see a signal. To this end the classical architecture which has been used for tens of years includes a single light source (generally a light-bulb) collimated by an optical system (that can be a single lens or a Fresnel lens in some applications). Then a light diffusing distributor that matches the size of the output beam is inserted at the output area to spread the light in the required directions.
This distributor can be realized by some divergent lenses if the pattern is smooth and simple, or—when fine shaping is needed—with several refractive elements or a diffusive surface. The distributor is divided in different zones, each zone deflecting the light in a specific direction. The power balance between each angular sector is obtained by a surface repartition between the diffuser zones.
A drawback of this configuration is that for each situation (right curve, left curve, strait part . . . ) a specific distributor needs to be designed and used. This results in a large number of configurations to be anticipated and managed. Moreover, if different signal diameters are needed, each distributor must be recalculated and re-adapted to the new beam diameter. A further disadvantage of the above described configuration is that due to the zone division of the refractive distributor and depending on the angular position of the viewer, only a part of the surface of the distributor seems to be luminous, which leads to an inhomogeneous and sometimes non-circular light distribution on the signal surface. This effect is particularly visible when getting close to the signal, i.e. when the user is close enough to be able to distinguish the different zones of the distributor. Moreover, the lifetime and the power efficiency of light bulbs are quite poor and lead to high maintenance costs.
More recently, to overcome the maintenance limitation of light bulb based device, a luminaire using multiple source based on LED (Light Emitting Diode) have been used. However, when safety is necessary (which is the case for railway applications), each source must contribute to the whole distribution pattern, i.e. the light of each source has to be spread to the same relative light intensity distribution, so that a source failure does not lead to a variation in the relative light distribution. This means that in order to spread the light in different directions, each collimated source must be equipped with an individual distributor that provides the desired light distribution (THALES “FieldTrac 6365 LELD-Signal”). By using this configuration, there is no need to redesign the light distributors if the size of the signal is to be adapted: the surface variation can be compensated by adding or removing collimated (and distributed) light sources. However, by doing so, the electrical design of the signal can change significantly which also has an impact on the optical output flux of the luminaire, both limiting the scalability of this architecture.
Compared to a light bulb the LED active surface is rather large, meaning that individual collimation of each LED will lead to a more diverging beam. In other words LEDs suffer from low brightness requiring the use of tens of LED, thus leading to more complex and costly architectures. Additionally, because of the small size of the distributors, it is more difficult to shape a sharp angular distribution.
Due to the described limitations, the light distribution of this kind of signal is far less efficient than the one achievable with single light sources. This means that here again, the light distribution must be optimized for each specific situation (right curve, left curve, strait part . . . ), meaning that specific distributors are to be designed and used in order to save available optical output flux of the luminaire.
It is an object of the invention to provide a low maintenance traffic light luminaire which generates a homogeneous light distribution for a variety of trail arrangements (right curve, left curve, straight line) independent of the distance of the viewer.